The present invention relates to electric and electronic detonators and, more specifically, to such detonators having an increased voltage requirement for firing, in order to provide protection against inadvertent firing by stray or induced electrical currents, magnetic fields of electrical conductors, radio signals, lightning strikes or the like.
Electric and electronic delay detonators are known in the art, including detonators which have electronic timing circuits therein. This enables setting electronic time delays between the receipt of an initiation signal and firing of the detonator. Such electric and electronic delay detonators are often provided with a test circuit and, for safety's sake, the energy used for testing is normally set at a level which is insufficient to initiate the igniter. This is usually accomplished by including a ballast resistor in series with the igniter so that the voltage drop across the resistor is great enough to insure that the voltage used to test the igniter is insufficient to activate the igniter. The resistor consumes as waste heat a substantial amount of the energy supplied to the detonator. Such detonators must therefore have an energy supply capable of both satisfying the voltage drop over the resistor and carrying out the testing. When the detonator is to be initiated, sufficient energy must be available both to run the timing circuit and, ultimately, to fire the igniter. This increased energy demand for testing and firing results in smaller shot sizes and a reduction in available delay times. This is because, obviously, larger shot sizes require more energy and longer delay times require the delay circuits to run longer, thereby consuming more energy.
In seismic applications, boreholes are typically primed well in advance of shooting the holes. An unattended primed borehole with a typical seismic blasting detonator may result in initiation of the blast by stray currents or by tampering. Even the energy available from a common flashlight battery connected across the exposed leg wires may initiate the detonator. The art has employed various methods to increase the voltage required to initiate a detonator in order to reduce the sensitivity to stray currents and tampering. However, such prior art methods also increase the energy required to initiate the detonators. Accordingly, there is a need in the art for a detonator ignition protection circuit that overcomes these drawbacks.